Stereological analysis of electron micrographs will be done on rat ventricular muscle during various normal and pathological growth processes. Under the same conditions, Cu and sequested myofibrillar Mg contents Cu67 exchange and Mg28-exchange will be measured. Morphometric analysis will include gap junctional area at the intercalated disk, areas of external sarcolemma, T-system plasma membrane, sarcotubular membrane, inner and other mitochondrial membrane, as well as fractional values of myofibrils, mitochondria, sarcotubules, T-system, nuclei, mitochondrial matrix, and intercristal space.